The present invention pertains to power amplifiers and in particular to a method and apparatus to protect power amplifiers from failure caused by operational conditions.
Radio frequency (RF) power amplifiers are known in the art for use in amplifying RF signals for broadcasting purposes, including radio and television. These amplifiers may be employed for broadcasting either analog or digital signals. In some applications, such as mobile communication systems, RF power amplifiers are frequently cycled, turned On and Off, during operation. The average power of these amplifiers is defined as the product of the duty cycle and the peak power. As the duty cycle of a RF power amplifier increases, so does its average power. As the average power of an amplifier increases it may be exposed to operating conditions which causes or accelerates its failure.
Typically, power amplifiers are designed to operate within certain current, voltage, temperature, and frequency limits. Operating a power amplifier beyond its designed limits exposes the amplifier to failure. Thus, power amplifier manufacturers typically specify the operating parameters or conditions for each particular type of power amplifier, including power dissipation and operating temperatures.
Manufacturers also typically over-design power amplifiers to withstand wide power and temperature variations. Designing such rugged power amplifiers tends to make them larger than otherwise necessary, more expensive, and may degrade their RF performance. Additionally, even ruggedly designed power amplifiers are susceptible to certain types of failures.
One such type of failure may occur when the output of the power amplifier is mismatched. That is, power amplifiers are often designed to operate with a particular impedance or termination value. For example, the output of a power amplifier may be designed to be coupled to a nominal 50 Ohm load or termination, such as an antenna with impedance of 50 Ohms. However, in an actual implementation, the power amplifier may be terminated with a substantially different terminating impedance value. Such mismatched termination may result in the power amplifier dissipating more power internally than what it was designed to dissipate. This may result in additional heat being generated which may cause the power amplifier to operate at temperatures higher than what it was designed to operate.
Another circumstance which may cause a power amplifier to fail may occur when the source and/or input voltage to the power amplifier changes during operation. That is, a power amplifier may be designed to operate within a certain voltage range; for instance 2.8 to 3.4 Volts DC. However, during operation, the source and/or input voltage to the power amplifier may increase beyond such range; for instance 5.0 Volts DC. Such increase in voltage may cause the power amplifier to dissipate more power internally, resulting in more heat being generated and increasing its operating temperature. This may result in the power amplifier operating beyond its designed limits and lead to its failure.
Accordingly, there is a need for a method and/or apparatus to protect power amplifiers from failure caused by mismatched loads and/or large supply voltages.